Magnetic-field induced shape memory effect in Ni2MnGa sputtered films

The shape memory effect (SME) by magnetic field is interesting and important for physics and application. The Ni2MnGa films were deposited on a poly-vinyl alcohol (PVA) substrate with a radio-frequency (RF) magnetron sputtering apparatus using Ni52Mn24Ga24 or Ni52.5Mn22Ga25.5 target. After separating from the substrate, the films were heat-treated at 1073 K for 36 ks for homogenization and ordering. They were constrained in a silica tube and heated at 673 K for 14.4 and 57.6 ks. The two-way SME for each constraint-aged film was confirmed by thermal cycling. The shape change of film under magnetic field was observed at constant temperature between As∗ and Af∗, or Ms∗ and Mf∗. Magnetic field was applied parallel to the film surface up to 5 T using a super-conductor magnet. These films showed the SME by magnetic field. The strain by magnetic field was dependent on the measurement temperature. At temperatures between Ms∗ and Mf∗, the strain increased with increasing magnetic field and slightly decreased with decreasing magnetic field. The martensitic transformation was induced by the magnetic field up to 5 T, which corresponds to that observed by cooling down to about 4 K.